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Is it important for large farms to perform sequencing of a local PRRSV strain before starting the vaccination program?

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Is it important for large farms to perform sequencing of a local PRRSV strain before starting the vaccination program?

Is it important for large farms to perform sequencing of a local PRRSV strain before starting the vaccination program?

Were there identified field strains heterologous to a MLV PRRS vaccine and where the vaccine lacked efficacy? Is there any risk for a farm to change the vaccine against PRRS? Is it important for large farms to perform sequencing of a local PRRSV strain before starting the vaccination program?

Answered by: Tomasz Stadejek I Published on: July 27, 2016

The level of cross protective immunity following natural infection, or following vaccination with modified live vaccine depends on the antigenic relationship of an immunizing and a challenge strains. Complete protection (sterile) is possible if the immunizing and challenging strain is the same.

Only in case of highly virulent strains the immunity against challenge with the same strain not complete immunity has been described, still, it was protective against clinical disease. It could be assumed that always the vaccine strain is heterologous to the field strain of a farm, and in effect the protection of vaccinated pigs against the challenge is only partial.

The level of cross protection will depend on the antigenic relationship of the vaccine and the wild type strain. In extreme cases the level of cross protection may be seen as not satisfactory. It has to be remembered that environmental conditions and other infectious factors can impact the level of immunity. Large quantities of PRRSV in the environment mean higher infectious dose for pigs. Co-infections with immunosuppressive agents at the time of vaccination with PRRSV vaccine or at the time of challenge can compromise the immune response.

Unfortunately there are no laboratory methods that could allow to predict efficacy of a given vaccine against a given field strain. DNA sequencing of a local strain in order to choose the vaccine does not make much sense. Performing genetic analysis of a strain most often we are looking at about 4% of the genome! Periodical sequence analysis of strains from a given farm or a production system is worthwhile undertaking as it allows to detect shifts in the circulating strains and to identify new variants. The latter can be helpful in explaining observed changes in vaccine efficacy. Swapping one vaccine for another does not pose any significant risk.

However, it has to be remembered that in some cases (errors in vaccination protocols, continuous flow production) vaccine related strains can persist in a farm. Co-circulation of non-pathogenic vaccine related strain(s) and pathogenic wild type strains may hamper correct interpretation of diagnostic results.